We have characterized four types of chloride channels at the single channel level in the small diameter, unmyelinated axons C fibers) of the garfish olfactory nerve. Cl channels were incorporated into planar lipid bilayers by fusing axon plasma membrane vesicles derived from the nerve with the bilayers. Currents were measured in symmetric 130 mM N-methyl d-glucamine Cl. Two types had small conductances and were sensitive to 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). One of these was outwardly rectifying, with a 46+-4 pS (n=12)conductance at positive and 24+-2 pS at negative voltages. The other had a linear current- voltage profile with a 45+-7 pS (n=10) conductance. A third type had a linear, 122+-30 pS (n=6) conductance, was DIDS-insensitive, and had subconductance states. The fourth type of Cl channel had a conductance of 410+-26 pS (n=3), was linear, had an open probability (po) near 1 over +-60 mV, had subconductance substrates, and was DIDS-insensitive. None were calcium-dependent. We conclude that the axon membranes of the garfish olfactory nerve contain at least four distinct types of Cl channels whose role in regulating the excitability of these small nerve fibers is unknown.